A backlighting device (backlight) serves for homogeneously illuminating a surface. One example is the illumination of a rear side or rear surface of a liquid crystal display (LCD). In a configuration designated as LED backlight, light emitting diodes (LEDs) are used for generating light radiation. A customary arrangement is the so-called direct backlight. In this case, LED light sources each having a primary optical unit are situated in a plane parallel to the surface to be illuminated. The primary optical units serve to provide an emission characteristic with a light emission taking place toward the side. This serves to obtain a homogeneous irradiation of the surface to be illuminated by means of a superimposition of the light proportions of neighboring light sources.
At the edge of the backlighting device it is necessary to enable, by means of a reflection of radiation, a superimposition of light radiation such that the surface to be illuminated experiences a homogeneous irradiation at this point, too. The reflection of radiation can take place at side walls of a wall structure extending around the light sources in a frame-shaped fashion.
One possibility is an orthogonal orientation of the side walls of the frame in relation to a plane predefined by the light sources, and a configuration of the side walls as a specular reflector. Light of a marginal light source can thereby be reflected in each case as if this proportion came from a further fictitious light source situated outside the backlighting device.
The concept mentioned above is simple, but not always desirable. In this regard, instead of a perpendicular orientation, an angled orientation of the side walls is required, rather, in order to obtain a flat appearance. This concerns the field of flat screens, for example.
A configuration with inclined and specularly reflective side walls leads to an increased luminance and hence a bright edge at the surface to be illuminated, such that homogeneous illumination is no longer possible. Conventionally, the surface of the oblique side walls is therefore made diffusively reflective. This is associated with a Lambertian emission characteristic, as a result of which part of the light impinging on the side walls can be reflected back again in the direction of the light sources.
The greater the intended distance between the light sources and the smaller the intended distance between the light sources and the surface to be illuminated, the more difficult it is, however, to obtain a homogeneous illumination with the aid of diffuse reflection. With the large distance between the light sources and a small distance between the light sources and the surface to be illuminated, the proportion of light reflected back is insufficient, with the consequence that an increased luminance occurs at the edge.